1. Field of the Invention
The invention relates to a polyurethane based adhesive used for manufacturing a polarizing structure. It also relates to a polarized ophthalmic lens comprising such polarizing structure.
2. The Prior Art
The optical or transmission properties of ophthalmic lenses can be modified by laminating a film onto a lens surface. These films are often referred to as functional films such as polarizing films which are capable of reducing glare while providing a high level of optical transmission.
There are many ways to make polarized lenses, such as by injecting a lens against a polarizing film, casting a lens adjacent a polarizing film, casting a lens containing a polarizing film, or laminating the film with a glue. It is known to manufacture a polarized optical lens by laminating a polarizing structure onto a lens surface. Typically, an adhesive layer, for example thermal or UV curable glue, holt melt adhesives (HMA) or pressure sensitive adhesive (PSA) is placed between one surface of the optical lens and the polarizing structure, so as to permanently retain the structure on the surface of the optical lens. Such polarizing structure is obtained by incorporating a polarizing film, for example a polyvinyl alcohol (PVA) polarizer between two protective films. Materials for the protective layers can typically include cellulose triacetate (TAC), polycarbonate (PC), cellulose acetate butyrate (CAB), poly(methyl methacrylate) PMMA or poly(ethylene terephthalate) (PET). The polarizing structure is laminated together with a water based adhesive between the protective films and the PVA film, such as a polyvinyl alcohol (PVOH) based adhesive. This adhesive can be sensitive to water; therefore, the TAC/PVA/TAC structure has very poor wet adhesion and can be easily separated when contacted with water.
Because of the water sensitivity of this polarizing structure and its poor wet adhesion, the polarized laminated lens comprising such structure has delamination problems during wet wheel edging during processing of polarized lenses. More particularly, it has been seen that the TAC-PVA-TAC film ensemble is separated at the edge of the lens due to the combined effects of wheel edging force with water during the edging process.